Ziegler-Natta (ZN) type polyolefin catalysts are well known in the field of polymers, generally, they comprise (a) at least a catalyst component formed from a transition metal compound of Group 4 to 6 of the Periodic Table (IUPAC, Nomenclature of Inorganic Chemistry, 1989), a metal compound of Group 1 to 3 of the Periodic Table (IUPAC), and, optionally, a compound of Group 13 of the Periodic Table (IUPAC) and/or an internal donor compound. ZN catalyst may also comprise (b) further catalyst component(s), such as a cocatalyst and/or an external donor.
Various methods have been developed for preparing ZN catalysts. In one conventional method a supported ZN catalyst system is prepared by impregnating the catalyst components on a particulate support material. For example in WO-A-01 55 230 of Borealis, the catalyst component(s) are supported on a porous, inorganic or organic particulate carrier material, such as silica. In a further well known method the carrier material is based on one of the catalyst components, e.g. on a magnesium compound, such as MgCl2. This type of carrier material can also be formed in various ways: EP-A-713 886 of Japan Olefins describes the formation of Mg solution by melting MgCl2 with an alcohol which is then emulsified and finally the resultant mixture is quenched to cause the solidification of the droplets. Alternatively, EP-A-856 013 of BP discloses the formation of a solid Mg-based carrier, wherein the Mg component containing phase is dispersed to a continuous phase and the dispersed Mg phase is solidified by adding the two-phase mixture to a liquid hydrocarbon.
The formed solid carrier particles are normally treated with a transition metal compound and optionally with other compounds for forming the active catalyst.
Accordingly, in case of above external carriers, the morphology of the carrier is one of the defining factors for the morphology of the final catalyst. However also the subsequent surface treatment of the particles may have an impact on the morphology of the final supported catalyst.
One disadvantage encountered with the supported catalyst systems is that the surface treatment (impregnation step) of the support with one or more catalytically active compounds may lead to non-uniform distribution of the active component(s) and in turn to an inhomogeneous polymer material.
WO-A-00 08073 and WO-A-00 08074 of Borealis describe a further method for producing a solid ZN catalyst, wherein a solution of a Mg-based compound and one or more further catalyst compounds are formed and the reaction product thereof is precipitated out of the solution by heating the system. Furthermore, EP-A-926 165 of Borealis discloses another precipitating method, wherein a mixture of MgCl2 and Mg alkoxide is precipitated together with a Ti compound to give a ZN catalyst.
EP-A-83 074 and EP-A-83 073 of Montedison disclose methods for producing a ZN catalyst or a precursor thereof, wherein an emulsion or dispersion of Mg and/or Ti compound is formed in an inert liquid medium or inert gas phase and said system is reacted with an Al alkyl compound to precipitate a solid catalyst. According to examples said emulsion is then added to a larger volume of Al compound in hexane to cause the precipitation.
In EP-A-258 089 of Montedison an emulsion of a catalyst components, or a precursor thereof, comprising a Mg and/or Ti compound is formed in perfluoropolyether and the dispersed phase is reacted with a reducing and/or halogenating agent to precipitate said catalyst components or a precursor thereof. According to the examples said emulsion is then added to a larger volume of a solution of a halogenating agent and TiCl4 treatment is further continued in the formed suspension to effect the surface treatment of the particles.
In general, a drawback of such precipitation methods is the difficulty to control the precipitation step and thus the morphology of the precipitating catalyst particles.
Furthermore, the precipitation of the catalyst component(s) may often proceed via a “tar-like” intermediate stage. Said undesired sticky precipitate agglomerates easily and sticks to the walls of the reactor. The morphology of the catalyst would then of course be lost.
Furthermore, in addition to above heterogeneous catalyst systems, homogeneous catalyst systems, wherein the catalyst component(s) are used in a form of a solution in the actual polymerisation process are also known in the art. For instance, EP-A-536 840 of Enichem discloses i.a. the preparation of a catalyst emulsion by contacting in an inert diluent a Mg dialkyl and Al trichloride. This emulsified precursor is then contacted with at least one compound of a transition metal to obtain the final catalyst in the form of an emulsion. The emulsion is used as such for polymerisation of olefins.
The use of homogeneous catalysts in industrial applications has been limited due to their fast deactivation behaviour. In case of emulsions also stability problems may occur.
Accordingly, although much development work has been done in the field of Ziegler-Natta catalysts, there remains a continuing search for alternative or improved methods of producing ZN catalysts with desirable properties.